1. Field of the Invention
The present invention relates to a method of forming photonic crystals and, more particularly, to a method of forming photonic crystals using a semiconductor-based fabrication process.
2. Description of the Related Art
A photonic crystal is a spatially-periodic dielectric structure that reflects electromagnetic radiation that falls within a range of frequencies, and passes radiation that falls outside the range of frequencies. The range of frequencies, in turn, is defined by a number of factors, including the center-to-center spacing of the structure and the dielectric constants of the materials used to form the structure.
FIG. 1 shows a perspective view that illustrates a first example of a prior-art photonic crystal 100. As shown in FIG. 1, crystal 100 includes a block 110 that has a first side 112, an opposing second side 114, and a number of first openings 116 formed through block 110 that extend from first side 112 to second side 114.
Block 110 also has a third side 120, an opposing fourth side 122, and a number of second openings 124 formed through block 110 that extend from third side 120 to fourth side 122. In addition, openings 116 and 124, which are perpendicular to each other, are formed in alternating layers. Further, block 110 has a fifth side 126 and an opposing sixth side 128.
In the example shown in FIG. 1, block 110 is a dielectric material and therefore has the dielectric constant of the material, while openings 116 and 124 are air filled and therefore have the dielectric constant of air (openings 116 and 124 can also be filled with other materials). Further, openings 116 and 124 have a center-to-center spacing 130.
In operation, when electromagnetic radiation is incident on fifth side 126, crystal 100 allows frequencies outside of a range to propagate through crystal 100 and exit from sixth side 128, while at the same time stopping frequencies within the range from propagating through crystal 100.
FIG. 2 shows a perspective view that illustrates a second example of a prior-art photonic crystal 200. As shown in FIG. 2, crystal 200 includes a block 210 that has a first side 212, an opposing second side 214, and a number of first rods 216 formed through block 210 that extend from first side 212 to second side 214.
Block 210 also has a third side 220, an opposing fourth side 222, and a number of second rods 224 formed through block 210 that extend from third side 220 to fourth side 222. In addition, rods 216 and 224, which are perpendicular to each other, are formed in alternating layers. Further, block 210 has a fifth side 226 and an opposing sixth side 228.
In the example shown in FIG. 2, rods 216 and 224 are formed from a first dielectric material and therefore have the dielectric constant of the first material. The space between rods 216 and 224, however, is filled with a second dielectric material and therefore has the dielectric constant of the second material. Further, rods 216 and 224 have a center-to-center spacing 230. Crystal 200 operates the same as crystal 100.
One of the difficulties with photonic crystals is that photonic crystals are difficult and/or expensive to manufacture. For example, crystal 100 is formed by drilling openings 116 and 124 through block 110. Drilling, however, has inherent size limitations with regard to the maximum thickness of block 110, the minimum diameter of openings 116 and 124, and the minimum spacing between openings 116 and 124.
On the other hand, crystal 200 requires a number of layers and a large number of processing steps for each layer. For example, the first layer of crystal 200 can be formed by depositing a layer of first material, and masking and etching the first layer to form first rods 216. After this, the second material is deposited to fill up the gaps between first rods 216, and is then planarized to form the first layer. These steps must then be repeated for each layer in crystal 200.
Thus, there is a need for a method of forming photonic crystals that is not subject to the size limitations of drilling and requires substantially fewer processing steps.
The present invention provides a method for forming a photonic crystal using a semiconductor-based fabrication process. Current-generation semiconductor fabrication processes are capable of producing deep sub-micron device sizes. As a result, the photonic crystal of the present invention can be formed down to sub-micron sizes. In addition, the present invention requires relatively few processing steps, and can be formed as part of an integrated circuit that includes circuitry which responds to the electromagnetic radiation received by the crystal.
In accordance with the present invention, the method begins by forming a first layer of material over a substrate. The first layer of material has a first dielectric constant. Next, a second layer of material is formed on the first layer of material. The second layer of material has a second dielectric constant.
Following this, the forming the first layer step and the forming the second layer step are repeated a predetermined number of times to form a multi-layered structure with alternating layers. The multi-layered structure has a top layer and a plurality of underlying layers. The top layer has a top surface.
Next, the top layer and the underlying layers are etched to form a plurality of photonic stacks and a space between the photonic stacks. The plurality of photonic stacks have a plurality of top surfaces. After this, a layer of interstack material is formed over the substrate to fill up the space between the photonic stacks.
The present invention also includes a photonic crystal that is formed on a semiconductor substrate. The photonic crystal includes a plurality of spaced-apart photonic stacks that are formed over the receiving region of the substrate. The photonic stacks have top surfaces.
Each photonic stack has a plurality of layers of material that alternate between a first layer of material and a second layer of material. The first layer of material has a first dielectric constant, while the second layer of material has a second dielectric constant. The photonic crystal also includes an interstack material that is formed over the substrate between and adjoining the plurality of photonic stacks.